The long-range goal of this project is to understand how descending neural systems modify information processing in the cochlea and cochlear nucleus. Existence of descending innervation to hair cell systems in most vertebrate species examined is strong presumptive evidence for the importance of descending systems. There is evidence that the function of these systems may be: 1) to adjust the best operating range of the inner ear, 2) to reduce the effects masking noise, and 3) to protect the inner ear from over stimulation by intense sound. These functions have direct health relevance; their interruption would severely damage important processes like speech discrimination and render the inner ear more vulnerable to damage. [unreadable] [unreadable] This project will concentrate on one particular descending system, the medial olivocochlear (MOC) neurons. MOC neurons respond to sound and form a reflex that sends information to the outer hair cells of the cochlea. We know much about the afferent limb of the reflex, the auditory-nerve fibers, and recently we have made much progress on the efferent limb of the reflex, the MOC neurons. However, the middle segment of the reflex, which passes through the cochlear nucleus of the brainstem, is not understood. The experiments in guinea pigs will identify the MOC reflex interneurons of the cochlear nucleus, by using lesions and electrical stimulation to determine their location (Aim 1). These experiments will answer questions like, "Are the MOC reflex interneurons located in a particular subdivision of the cochlear nucleus?". and "Is this location the same when the MOC reflex is elicited by ipsilateral vs. contralateral sound?" We will also record and determine which types of cochlear nucleus units have responses that are comparable to those of single MOC neurons, thus predicting which unit type and subtype are the interneurons (Aim 2). Finally, we will determine which classes of cochlear nucleus neurons send projections to MOC neurons, and study the kinds of synapses that they form on MOC neurons. Overall, these studies will answer the question, "Which of the many parallel pathways of the auditory brainstem leads to the descending olivocochlear neurons?" The knowledge will enable us to precisely and clearly specify the central pathway of the MOC reflex. It will also greatly increase our understanding of both the descending and the ascending auditory pathways. [unreadable] [unreadable]